Known in the art are processes for producing pulp which comprise impregnation of vegetable raw materials such as wood chips with a solution of nitric acid, cooking in a vapour-gas or liquid medium, followed by alkali extraction.
Concentration of nitric acid is varied within the range of from 3 to 42% by weight and the impregnation temperature depends on concentration of acid and is varied within the range of from 12.degree. C. (for a concentrated acid) to the boiling temperature (in the case of diluted solutions). Duration of impregnation is determined by the technology employed and is varied from several minutes to 18 hours and more.
To accelerate the process of impregnation of the vegetable raw materials and ensure a uniform distribution of nitric acid therein, various methods for intensification of this operation have been developed.
These are exemplified by impregnation of the vegetable stock with acid in vacuum, setting wood chips under vacuum prior to supply of nitric acid into the apparatus, followed by elevation of pressure up to 3 atm.g; disintegration of the vegetable stock and the use thereof in the form of thin shavings as well as keeping of the impregnated vegetable stock at a temperature within the range of from 40.degree. to 80.degree. C. for a period of from 3 to 5 hours to equalize the concentration of the initially absorbed acid over the entire volume of the vegetable stock. There is also a method comprising mechanical compression and decompression of a vegetable stock submerged into a solution of nitric acid thrice elevating and diminishing the pressure in alternation.
However, the use of said operations prior to impregnation of the vegetable stock with a solution of nitric acid does not ensure a required uniformity of distribution of acid in the vegetable stock which causes a substantial amount of a non-cooked product (wastes) lowering the yield of the final pulp.
Furthermore, the resulting pulp has an insufficient mechanical strength and a low viscosity of its cuprammonium solutions.
In the prior art processes for the production of pulp the vegetable stock impregnated with a solution of nitric acid is subjected to cooking in a liquid or vapour-gas medium. It is advisable to use the following conditions, namely: maintaining the vegetable stock under atmospheric pressure at a temperature of from 96.degree. to 99.degree. C. for a period of from 0.5 to 1.5 hour. At the temperature of 98.degree.-99.degree. C. there are formed about 50.sup.3 m of gaseous products per 1 ton of the vegetable stock. The volume of the gaseous mixture formed during the cooking is decreased with decreasing the process temperature; however, the cooking duration is increased from 7 to 10 hours.
The main disadvantage of the prior art processes for the production of pulp upon cooking under the above-described conditions is the formation of pulp containing a considerable amount of hemi-cellulose which exerts a detrimental effect on the quality of the final pulp product. Furthermore, in the prior art processes for the production of pulp there is lack of processes of decontamination of the gaseous products of the cooking which results in pollution of the environments.
Also known in the art is a process for producing pulp from a vegetable stock including steaming effected by passing a live steam through a bed of the vegetable stock for 0.3-0.5 hour, impregnation of the vegetable stock with nitric acid having concentration of from 6 to 15% by weight at a temperature within the range of from 40.degree. to 50.degree. C. under atmospheric or an overatmospheric pressure; cooking in a liquid or vapour-gas mixture at a temperature of 96.degree.-99.degree. C. accompanied by liberation of toxic products, and an alkali extraction. The alkali extraction is effected by means of aqueous solutions of caustic soda, potash, soda, ammonia, calcium hydroxide or a mixture thereof. This process makes it possible to produce pulp with a yield of up to 45% by weight and containing up to 89% of alpha-cellulose and down to 12% of hemi-celluloses. This pulp, however, does not possess the required quality parameters.
This prior art process has the following disadvantages: a long duration of the impregnation stage, non-uniformity of impregnation of the vegetable stock with nitric acid resulting in a large amount of under-cooking, a high content of hemi-celluloses, absence of decontamination of the gaseous cooking products.